The automated deposition of coating materials, such as adhesives, caulks, or sealants onto the surfaces of workpieces is commonly performed through the use of program control devices, such as robot-mounted fluid dispensing guns. The devices which support the guns are programmed to move the guns through a predetermined path with respect to a workpiece surface which corresponds to a desired pattern of application of the fluid onto the surface. In such devices, a control program establishes the tool speed, while a fluid dispensing control controls the discharge of fluid. The fluid is to be dispensed in accordance with an operator defined input signal which defines a desired physical characteristic of the applied fluid. For example, the input signal may represent bead size which defines the desired diameter of the bead to be applied to the workpiece. To achieve the desired bead size, the rate at which fluid is dispensed from the gun nozzle must be proportional to the relative velocity between the workpiece and the dispensing gun. Therefore, the rate at which fluid is dispensed through the gun nozzle must vary proportionally in real time in response to changes in the tool speed signal. The tool speed is defined as the linear or scalar speed at which the point of application of coating material on the workpiece surface moves with respect to the workpiece surface. The flow rate of fluid through the dispensing gun can be controlled by measuring the pressure drop across the nozzle of the dispensing gun and controlling the operation of a metering valve regulating the flow of fluid through the gun.
The above fluid dispensing process is further subject to unpredictable changes in the flow characteristics of the fluid being dispensed. For example, changes in temperature, and other conditions will change in real time the flow characteristics of the fluid being dispensed; and those changes in flow characteristics will change the flow rate and hence the volume of fluid dispensed. In addition, there are flow non-linearities introduced by the shear effects of the fluid flow through the dispensing nozzle; and those flow non-linearities are dependent on the nozzle and nozzle wear. Therefore, it is desirable that the volume of fluid dispensed over a dispensing cycle be a controlled variable, and the total volume of fluid dispensed each dispensing cycle is measured.
As disclosed in the Baron, et al. U.S. Pat. No. 5,065,695 issued to the assignee of the present invention, the fluid dispensing control compensates the tool signal by a correction factor that is determined as a function of the changes in viscosity caused by shear effects of the fluid through the nozzle. As part of a setup calibration procedure, the flow of fluid through the nozzle is measured in response to different tool speed signal settings thereby producing a table data values which are stored in the fluid dispensing control memory. The stored data is used to calculate an interpolated linearization factor which is applied to the adjusted tool speed signal. The stored linearization factor is correlated to the relationship between flow rate and nozzle pressure as measured during the calibration process. However, the stored data remains fixed, and hence, the compensation is fixed over many dispensing cycles even though the relationship of flow rate to nozzle pressure may change. While the change is compensated for in a volume measurement control loop, the above system has the disadvantage of not being more quickly responsive to changes in the flow rate-nozzle pressure relationship.
In addition, the volume of fluid measured during one dispensing cycle is compared to a volume set point, and a material volume error signal is produced that represents changes in material viscosity that are caused by temperature changes or other dynamic conditions. The material volume error signal provides a compensation for changes in material viscosity that are caused by temperature changes or other dynamic conditions. The material volume error signal is produced from a proportional and integrating comparator. The volume of material that is dispensed is compared to a material weight setting, that is, a volume set point to produce a material volume error signal. Within the proportional and integrating comparator, a proportional term is set equal to approximately one-half the error signal; and the integral term is equal to the difference between the proportional term and the prior integral term. Consequently, the material volume error signal changes the pressure command signal gradually over several dispensing cycles to bring the volume of material that is being dispensed into conformity with the volume set point. For example, five or more dispensing cycles may be required to effect the volume compensation. While the above described system performs the necessary compensation, a disadvantage of the system is that several dispensing cycles are executed before the compensation is complete.
With the above system, the volume set point is determined by a preproduction experimental process in which a sample part is fixtured in the proximity of the fluid dispensing nozzle to simulate a production situation. The dispensing cycle is then executed, and the fluid dispensing nozzle and the workpiece are moved relative to each other such that the fluid is applied to the sample part in the desired pattern. Several dispensing cycles and parts may be required until the dispensed bead visually appears to be correct. When the correct bead is identified, the volume flow meter for that particular dispensing cycle is read; and the value of the volume flow meter is utilized as the material volume set point. Thereafter, the volume set point is conveyed to the production environment as part of the fluid dispensing program associated with that part.
There are several disadvantages with the above experimental process for determining the material volume set point. First, the experimental process requires that fluid be dispensed on workpieces that most probably are not usable in subsequent production. In addition, the volume set point is a part dependent parameter that must be carried with the other part related information adds to the complexity and cost of the overall system. Second, the experimentally determined material volume set point is a function of the flow characteristics of the fluid being dispensed during the test cycle. The flow characteristics of the fluid being dispensed during a subsequent production cycle may be different; and therefore, the volume represented by the previously determined volume set point may require further compensation in the production environment.